How to create an 'instance of' -like query in JPA 2.0?

问题

Say we've got an abstract @Entity Animal, and several entity classes that extend Animal, including Dog, Cat, Monkey and Bat.

How can I filter the results based on the extending entity's class?

Example: There are checkboxes where the user can select which entities to retrieve.

[ ] Dog
[X] Cat
[X] Monkey
[ ] Bat

Now I want to retrieve the entities with a (Named)Query defined in the Animal class. What kind of query parameters can I put into the query so that only the Cat and Monkey objects will be returned?

回答1:

I'm not absolutely sure it's supported by JPA, but the way to do it in Hibernate, regardless of the inheritance strategy, and thus even if you don't have a discriminator (or didn't map it as a property) is to use the implicit class property :

String jpql = "select a from Animal a where a.class in (:classes)";
Query q = em.createQuery(jpql).setParameter("classes", 
                                            Arrays.asList(Cat.class, Monkey.class));

EDIT :

I just found it's possible in JPA2 using the TYPE operator :

String jpql = "SELECT a FROM Animal a WHERE TYPE(a) IN :classes";
Query q = em.createQuery(jpql).setParameter("classes", 
                                            Arrays.asList(Cat.class, Monkey.class));

回答2:

You can use the discrimnator column and value to only search for certain subtypes of a given type. By default the discriminator column's name is DTYPE in JPA,the type is String and the value is the name of the class. You can however override this by adding the class level annotation @DiscriminatorColumn(name="KIND", discriminatorType=DiscriminatorType.INTEGER) (for the discriminator column's name and type) and @DiscriminatorValue("1") (for the specific discrimiminator value for a certain class). You can then use this in the WHERE clause of yoru JPQL query to only fetch certain subtypes, like: WHERE DTYPE="Dog" OR DTYPE="Cat"

来源：https://stackoverflow.com/questions/7807608/how-to-create-an-instance-of-like-query-in-jpa-2-0